Starch-granules are best examined in water; and a small quantity only of the starch must be placed on the slide, if the structure of the granules is to be seen clearly. They may be mounted in glycerine, although this makes them very transparent.
To those who possess a polariscope, starch-granules are particularly interesting, as they exhibit a black cross, and, with a plate of selenite laid beneath the slide, a beautiful play of colours.
In addition to the starch and chlorophyll, the cells of plants contain other matters, as gum, sugar, &c.; but as they are dissolved in the cell-liquid, they are not visible. In the cells of certain plants, however, spherical globules, with light centres and black outlines, will be met with: these consist of oil.
Raph´ides.—Lastly, occurring in the cells of plants, especially such as are soft and juicy (succulent), will be found minute, hard, colourless crystals, called raphides (ῥαφὶς, a needle). These are most frequently needle-like or acicular (acus, a needle), but sometimes prismatic or rod-like with flat sides; they are also not unfrequently grouped into little tufts. They may be readily found in a piece of the stem of garden-rhubarb ([Pl. I.] fig. 5 a), or of the common balsam.
Porous and spiral cells.—The walls of the cells of cellular tissue are sometimes covered with little dots ([Pl. I.] fig. 11 a), or slit-like markings; the cells are then called porous cells. A specimen of them may be obtained from a section of the pith of the elder (Sambúcus nígra).
Sometimes cells exhibit the appearance of a spiral line marking their walls, as if a little bell-spring were coiled up in them ([Pl. III.] fig. 2 a). These are called spiral cells, or spiral fibrous cells, and the tissue formed by them is called fibro-cellular tissue.
We now leave the cells of ordinary cellular tissue, to examine those in which the dimension of length predominates, so that they form tubular cells; and first of those required to possess strength and firmness, combined with flexibility. These qualities are met with in the cells constituting
Woody tissue.—Of this there are two forms, called respectively wood-cells and woody fibres.
The wood-cells are moderately long, more or less tapering and overlapping at the ends; and the cell-walls are thickened, so as to possess considerable firmness. These cells are found in the wood of stems, as in the white woody portion of an ash stick, that of a lime-tree, the stem of a Chrysanthemum, &c. ([Pl. I.] fig. 6). They are closely packed, and the tissue formed by their union is called prosen´chyma (πρὸς, close, ἔλχυμα, tissue).
In the other kind of woody tissue the cells are very long and slender, strong, yet flexible, gradually tapering at the ends, where they overlap each other; and they have thick walls, so that, when divided transversely, the cavity appears almost filled up ([Pl. I.] figs. 5 d, 9, & 7 b). This tissue is called woody fibre or pleuren´chyma (πλευρἀ, rib, ἔγχυμα), from its strength.